Scientific research papers are usually divided into four sections: Introduction, Materials and Methods (or Methods), Results, and Discussion. Whereas Introductions and Discussions should frequently use active voice and even first person active voice, the Methods section should not. Here are the reasons:

First, the actors (the researchers) are known and do not require or deserve highlighting when the paper describes what was routinely done.

We extracted DNA from the soil.

vs

DNA was extracted from the soil.

This difference seems small until one considers a long sequence of actions - see the next point.

Second, Methods often describe a sequence or list of actions. If each item in the list starts with we or I, the repetition is annoying and ineffectively emphasizes the actor.

We obtained ribosomal DNA sequences from isolates of nematode-trapping fungi from BMR and other California sites. To do this, we removed the fungi from storage, transferred them to potato dextrose agar, and allowed them to grow vigorously under sterile conditions for 3 weeks at 22ºC. We then scrapped hyphae from the agar surface with a sterile spatula and placed the hyphae and CTAB extraction buffer in a 1.5 ml centrifuge tube. We ground the hyphae with a micropestle and extracted DNA following the procedure of Jones (1897). We performed PCR with high resolution Taq polymerase and with forward primers ITS1F, LROR and LR3R and reverse primers.

vs

Ribosomal DNA sequences was obtained from isolates of nematode-trapping fungi from BMR and other California sites as follows. The fungi were removed from storage, transferred to potato dextrose agar, and allowed to grow vigorously under sterile conditions for 3 weeks at 22ºC. The hyphae were then scrapped from the agar surface with a sterile spatula and placed in a 1.5 ml centrifuge tube containing CTAB extraction buffer. The hyphae were ground with a micropestle, and DNA was extracted following the procedure of Jones (1897). PCR was performed with high resolution Taq polymerase and with forward primers ITS1F, LROR, and LR3R and reverse primers.

Third, random shifts between active and passive voice are illogical. Some writing text books recommend switching between active and passive voice for 'variety' but variety seems a poor reason to me.

Fourth, Methods sections usually use passive voice by convention, i.e., because so many papers have used passive voice for Methods, readers now expect it and accept it.

Although passive
voice should be the default voice in the Methods section,

a)
passive voice should not be the default in other parts of the paper.

b)
use first person, active voice in the Methods when the actor/researchers
(or their thoughts, inferrences, or assumptions) are important.

Because deer had never been observed in the forest, it was assumed that deer were not responsible for the herbivory.

vs

Because deer had never been observed in the forest, we assumed that deer were not responsible for the herbivory.

c)
some journals encourage writers to use active voice in all sections, including
the Methods - read the Instruction to Authors for the journal you intend
to submit your paper to.

Whereas I strive
for grace and occassional stylist flourish in other sections of the paper,
I settle for clarity, organization, and concision in the Methods. Trying to
convert Methods into scienfic poetry is a fool's task. Save your I's and We's
for the Introduction and Discussion, and even then you will run across editors
and reviewers who have learned that first person, active voice is unscientific.